The polymerization of monomers, especially olefins, can be carried out at a low pressure by using transition metal based catalysts, for example of the Ziegler-Natta type, which are catalyst compositions made up of a transition metal compound, an organometallic compound, and possibly other components. Ziegler-Natta type supported catalysts are prepared through the deposition of one or more transition metal compounds on a possibly pre-treated support, and the reduction of the thereby obtained compound by an organometallic compound which acts as a co-catalyst. The heterogenous catalysts thereby obtained are superactive, i.e. their activity is up to 20-100 times greater than the activity of unsupported catalysts.
The deposition of a transition metal compound on the support surface is carried out between a support in a solid phase and a transition metal compound usually in a liquid phase or in a liquid state. This stage of catalyst preparation thus involves operations for feeding the initial substances into the reactor and for creating in it suitable reaction conditions, for separating the unreacted liquid from the product, and for washing and drying the product.
The support on which the transition metal compound has been deposited is usable as a dry powder for olefin polymerization, in which case the reduction by an organometallic compound is carried out either before or at the beginning of the polymerization operation. On the other hand, the drying stage can be omitted and the reduction and the polymerization can be carried out immediately after the deposition on the support. The last-mentioned process is advantageous, for example, when an after-treatment is carried out on the supported catalyst by prepolymerizing a suitable monomer on its surface in order to improve its stability and its physical properties.
The above preparation stages are in general carried out in separate units, and thus the products have to be transferred from one unit to another during the preparation. Since both the reaction components and the reactions are highly sensitive to impurities, such transfer operations may cause weakening and unevenness of the catalyst quality. In addition, the various successive activation and washing operations require that the formed solid precipitate settles on the bottom of the tank, in which case the mixing must be discontinued and the liquid on top of the precipitate must be removed by siphoning. Such an operation leaves a large amount of activation or washing solution in the precipitate, and consequently its separation and washing efficiency is low and requires very many washing rounds in order to achieve the desired degree of purity. Thus, unreasonable amounts of time and washing chemicals are spent in the present-day operations and, furthermore, during the siphoning stage a proportion of the usable solids is lost together with the liquid which is removed.
The drying stage may be rather problematic, since the polymerization catalyst, which is sensitive to the effects of the oxygen and humidity of air, may be destroyed when the protecting liquid layer is removed, unless the operation is carried out in a highly inert, dry and oxygen-free atmosphere. When mechanical dryers are used, there is the risk that the fragile catalyst particles will break, and in vacuum drying unnoticeable air leaks are easily produced which will destroy the catalyst.